The present invention relates to a magnetic position measurement system with field containment means. The concept of using transmitting and receiving components with electromagnetic coupling is well known with respect to biomechanics and medical diagnostics, wherein a sensor assembly is mounted on a point of interest and the position of the point is determined relative to a fixed transmitter. This information is then used by computing systems to precisely show the relative motions of the points in question, which, in the medical sense, allows instruments to be precisely located in a human body with respect to the body and each other. This allows new, advanced methods of surgery and diagnostics to be performed.
When conductive materials are present, which is often the case on, below, or near an operating table, they generate eddy current fields, which distort the received magnetic field waveform, which distorts the output of the system unless the system utilizes some distortion reducing or compensating technique. When permeable materials are present, they bend and otherwise distort the magnetic field, with effects similar to conductive materials. In a surgical theater, both conductive and permeable materials are present in substantial quantities. They are a major component of many operating tables, surrounding equipment such as carts and equipment, and are present in the movable spot lamps used to illuminate the surgical field. Many operating tables have many degrees of positional and angular freedom to allow optimal placement of the surgical field relative to the surgeon, and are designed to be extremely stable and sturdy while supporting a heavy human body. As a result of these requirements, the tables contain numerous mechanisms allowing fore, aft, up, down, sideways, roll, and tilt motions. These mechanisms are physically robust and typically fabricated from steel, so that they have substantial field distortion characteristics. Shapes may include screws, rack and pinion gears, or scissors type actuators. The table surface may be one piece, or may be divided into several sections, with each section capable of motion relative to the other sections, to allow a body to be flexed such that various stresses and relative anatomical positions are optimal for a particular surgical or diagnostic procedure. The installed bases of operating tables are extremely diverse in design, and as the tables are often in service for many decades, there are many vendors, with each vendor carrying a number of different operating table designs. This poses a significant problem for magnetic position tracking systems which are used in a critical surgical environment. The operating volume for the tracker is typically within the body which lies on top of the table. This means that the tracking system is operating in close proximity to the metallic structures on, under, and around the table. The magnetic fields are distorted by these structures, which may result in large errors in the reported magnetic sensor position. The large diversity in table designs makes it impossible to predict the severity of distortion experienced on a given table. This is an unacceptable condition for a surgical environment. Attempts to compensate for these degrading effects have been made with varying degrees of effectiveness.
One method already employed is to map the entire operating volume each time the system is used. This is very time consuming and expensive, as potentially thousands of points must be taken in a precise manner if the distortion is severe and the operating volume large. It is also unreliable since during a surgical or diagnostic procedure, the table geometry is often changed which changes the relation of the table metallic structures relative to the tracking system, thereby requiring a new map if errors cannot be tolerated. Instruments and diagnostic equipment are also introduced and removed from the vicinity of the tracking system, thus rendering a map ineffective. Also, for severe distortion, a map may become totally ineffective, as the system may, at two different physical sensor spatial points, determine the sensor to be at the same position. In this case, the output data is of minimal use.
Another known method commonly described in prior art is to use AC fields over a conductive ground plane. The ground plane attenuates the magnetic field below the plane to nearly zero, which has the benefit of making the system insensitive to metallic objects below the plane. In the case of a dipole transmitter, the xe2x80x9cmethod of imagesxe2x80x9d is used to compute the theoretical magnetic field vectors over the plane, which are then used to provide sensor position. This method has drawbacks. One is that near the ground plane, the magnetic field intensity is nearly zero, and the vector crossing angles are degraded, which seriously reduces system performance with respect to accuracy and noise. The net result is that the sensor must be kept a few inches above the plane. Also, the dipole must be located some distance from the ground plane in order to reduce signal losses and degraded vector crossing angles within the operating volume. For a 1 cubic foot volume, the bottom of the transmitter must be about 2 inches above the plane for acceptable performance. To compute the height at which a patient must be elevated if lying on the transmitter, the thickness of the transmitter must be added to this 2 inch figure. Transmitter size is determined by required signal level within the operating volume. Sensor coil size for minimally invasive surgical applications is about 1 mmxc3x975 mm in cross-section, which is very small. The requirement for precise, low noise operation at the extreme edges of the volume requires that a relatively large magnetic field magnitude be present in order to induce sufficient signal in the small coils. Transmitter size is largely dictated by how much field it must output. Since the transmitter is typically a cube, to obtain sufficient signal within a 1 cubic foot volume with a small receiver coil, the practical transmitter dimensions are on the order of 2 inches per side. We can now see that the effective transmitter assembly in this prior art teaching, including the ground plane, is 4 inches thick. In a surgical environment, the patient must be elevated to levels which a surgeon may find uncomfortable. In addition, extra padding may become necessary if the patient must lie flat on the table. Both the transmitter and the padding must be secured to the table. In short, the configuration is cumbersome and may not allow the patient to be positioned in an optimal manner.
Placing the transmitter above the operating volume is not desirable as it will potentially interfere with the surgical field. Also, as the transmitter is placed further from the ground plane, and if the dimensions of the ground plane are fixed to be a square of about 18 inches on a side, the ground plane becomes ineffective at reducing the effects of metallic objects near the operating volume. The metal housings of the surgical lighting equipment will have a greater distorting effect in the upper portions of the operating volume, as they are closer to both the transmitter and receiver. Equipment used during the procedure, including the operating table, will cause potentially life threatening distortion, which is an unacceptable condition.
Position determination depends on relative vector magnitudes from the transmitter coils. Distortion effects may again be removed by using a process such as mapping. As the magnitudes of the transmitted magnetic vectors from the transmitter coils become more similar, a given fixed amount of error in their determination will result in an increased error in position output. Again, considering the limiting case, if the magnitudes become equal then position determination is not possible. This combined effect of reduced angle of transmitted vector intersection and reduced difference in transmitted vector magnitudes is known to those skilled in the art as geometric dilution. Use of a conductive ground plane under the transmitter will cause geometric dilution. The severity of the geometric dilution is increased as the transmitter becomes closer to the ground plane, and is also increased as the receiver becomes further from the transmitter. Geometric dilution generally imposes a practical limit on how close the transmitter of a magnetic tracking system may be placed to a conductive ground plane. For a 1 cubic foot motion box, geometric dilution approaches unacceptable levels if the transmitter is placed closer than 2 inches from an infinite extent conductive ground plane. Geometric dilution is also present in non-dipole transmitter configurations, and the effects of its presence are similar.
The following prior art is known to Applicant:
U.S. Pat. No. 4,849,692 to Blood discloses a method of eliminating eddy current distortion effects, which are generated by conductive objects, such as the stainless steel table surface, and in other objects having large surface areas. The distortion effects of permeable metals are not addressed by this system. This means that steel structures in, around, and under the operating region of the system will distort the received magnetic fields and degrade system performance. In addition, large, thick sheets of conductive metals such as Aluminum have eddy current decay times which can exceed 200 milliseconds. If the system uses 3 time division multiplexed transmit axes plus one period where all axes are off in order to compensate for the earth""s field, as described in the preferred embodiment, this means that the update rate is xc2xc*(200 mS)=1.25 Hz. This is unacceptably slow for many applications.
U.S. Pat. No. 5,767,669 to Hansen, et al. describes methods for eddy current field compensation without the need to compensate for the Earth""s field effects. This system has no provision for reducing the effect of nearby permeable metals, nor does it address the drawback of requiring a slow update rate while operating near large, thick sheets of highly conductive metals.
U.S. Pat. No. 5,600,330 to Blood discloses a non-dipole loop transmitter-based magnetic tracking system. This system shows reduced sensitivity to small metallic objects in the operating volume, as the field from the smaller object will fall off as 1/r{circumflex over ( )}3 with r being the received distance from that object, while the field from the larger transmitting loops will fall off as 1/r{circumflex over ( )}2, which yields a reduced effect from the small metallic object. Large sheets of metal, however, can have an effective loop area larger than the magnetic transmitter loops, which diminishes this advantage in field fall off rate, which has the general effect of making the system quite sensitive to large metallic objects.
Also, metallic objects parallel to and near the transmitter loops produce very large eddy current magnitudes which reduces the signal level within the operating volume. In order to reduce the effects of metallic objects near the transmitter in this system, the transmit coils must be placed some distance away from the ground plane in order to reduce signal loss, which occurs when a loop of wire gets close to a conducting ground plane parallel with the plane of the loop. In the case of the planar transmitter configuration in this system, a planar ground plane may be placed some distance below the transmit coils. For zero distance, the magnetic field reduction within the operating volume is nearly total, so one must find a compromise between effective transmitter thickness, defined as the total thickness of the transmit coils, ground plane, and spacing between them, and signal loss. Also, due to the fact that the ground plane eddy current loop area is large with respect to a single transmit coil area, there is an additional degrading effect as the sensor gets further from the transmitter. The ground plane current distribution is similar no matter which transmit coil is operating. This means that the ground plane eddy current field vectors will be similar also. Since the field at any point within the operating volume is the vector sum of transmit coil field minus ground plane eddy field, and the ground plane field effective radius is larger than the transmit coil radius, we can see that the further we get from the plane of the transmitter, the more the field is determined by the ground plane currents. The net effect is that the vectors from the 3 transmit coils are less distinct, which makes the system more sensitive to noise and metallic distortion, as the system uses differences in the vector magnitudes and directions to determine position. As these differences become small, a small change on one of the vectors can result in a large apparent change of receiver position.
U.S. Pat. No. 5,752,513 to Acker, et al. depicts a system which is a subset of the system described by Blood ""330, and operation in all respects is identical with respect to non-dipole transmitter properties and metal sensitivity.
U.S. Pat. No. 5,550,091 to Fukuda, et al. depicts a system using a so-called xe2x80x9cHelmholtzxe2x80x9d arrangement to produce a controlled field within the operating volume. One disadvantage of this system is its bulk, requiring the operating volume to be surrounded by the xe2x80x9cHelmholtzxe2x80x9d coil assembly. A second disadvantage of this system is that, when placed upon a metallic object such as a steel table, the magnetic field from the transmit coils will be distorted inside of the operating volume.
U.S. Pat. No. 5,640,170 to Anderson discloses a method of positioning a dipole over a specially constructed spiral over a ground plane. The dipole transmitter in this system must be located over the center of the spiral ground plane assembly, which makes patient placement more difficult in a clinical setting, as this placement may interfere with the surgical field during certain procedures. The benefit of this method is that it is possible to locate the transmitter closer to the ground plane, and one does not need to use the xe2x80x9cmethod of imagesxe2x80x9d to solve for position, but the disadvantage of transmitter location over the spiral/ground plane assembly is very similar to the case of a ground plane only.
U.S. Pat. No. 5,198,768 to Keren depicts a surface coil array for use in NMR applications. The system does not determine position, and does not utilize any methods for reducing the effect of nearby metallic objects.
The present invention represents a radical departure from the prior art relating to such transmitting and receiving position and orientation devices insofar as it is capable of satisfying the requirement of insensitivity to metallic objects under and adjacent to the transmitter assembly without exhibiting the disadvantages of signal degradation.
The present invention relates to embodiments of a magnetic field position and orientation measurement system with means for substantially containing, confining and re-directing the magnetic field from one or more transmit elements such that the fields are attenuated in areas outside of the operating volume in areas where metallic objects are commonly found.
The present invention relates to devices for measuring the position of receiving antennae relative to transmitting antennae using magnetic fields. Particularly, although not exclusively, such devices are for measuring that position in six degrees of freedom, namely, motion or translation in three coordinate directions (location) and/or rotational motion above three coordinate axes (orientation), location being commonly defined by X, Y, and Z linear coordinates referring to three mutually perpendicular directions and orientation being commonly described by pitch, roll and azimuth angular coordinates above three mutually perpendicular axes usually coincident with the three mutually perpendicular directions. The number of transmitting axes multiplied by the number of receiving axes is at least equal to a desired number of measured degrees of freedom.
The present invention includes the following interrelated objects, aspects and features:
(1) In the preferred embodiment, a flux containment means is used to redirect the flux vectors such that they are enhanced inside of the sensor operating volume and decreased under and adjacent to the transmitter plane, which reduces the sensitivity of the system to metals under and near the transmitter. The flux vectors from the transmitters are distorted by the flux containment means in a stable and repeatable manner, thus it is possible to precisely and repeatably characterize the distorted field. Once the precise vector distribution from the transmitter assembly is known, solution of position and orientation from a receiving means is a straightforward task to those familiar with the magnetic position tracking art. One reliable method for accomplishing this vector characterization is to utilize finite element analysis to compute the magnetic field vectors from the transmitter. Another reliable method is to employ one of several so-called mapping techniques which are known processes to those familiar with the art.
(2) The preferred embodiment of the present invention teaches a method for creating a representative magnetic transmitter assembly with reduced sensitivity to metallic objects under and adjacent to the operating volume of the system. The preferred embodiment also reduces the geometric dilution effects of a conductive ground plane to levels which are no longer of concern. This reduction in geometric dilution yields a system which is substantially less sensitive to distortion caused by metallic objects within the operating volume while maintaining insensitivity to metallic objects below the transmitter and reduced sensitivity to objects adjacent to the operating volume. The transmit means may include a number of wire loops, solenoids, or permanent magnets arranged in convenient shapes and locations for determining the position of the receiver within the volume. While 3-axis transmitters may be used in the present invention, it is also feasible to use transmitter means consisting of three transmitters having any angular or spatial relationship therebetween provided that relationship is known and quantified. The thickness of the permeable attenuator is generally chosen such that the saturation flux density of the attenuator material is not exceeded. Some ferrites have a saturation flux density of a few hundred Gauss, while annealed iron materials have about 15,000 Gauss. Mu metal has a saturation flux density of about 7,000 Gauss. Analyzing the attenuator thickness combined with a transmitter means using finite element analysis will produce values for flux density within the attenuator. For relatively thin attenuators, flux density is inversely proportional to thickness, so if the density is seen to be at or near saturation, the attenuator can be made thicker. In other cases, the transmitter excitation may be reduced. If the flux density within the attenuator exceeds the saturation value, the shielding effect of the attenuator is reduced. In some applications in which cost or weight is placed at a premium, operation with a saturated attenuator may still be acceptable, as the attenuator will still exhibit reduced sensitivity to metallic objects adjacent to and below the transmitter compared to a non attenuator equipped system.
(3) The present invention achieves the requirement for a system which may be placed upon a surface of any extent and composition without degrading the accuracy of the position readings from a sensor located within the desired operating volume. It achieves this goal for both AC and DC transmitter excitations, which is not at all possible using prior art ground plane based compensation methods. It achieves this goal while significantly increasing the magnetic field intensity within the operating volume, which is not possible using prior art ground plane based compensation methods. It also avoids the problem of geometric dilution which is introduced when a conductive ground plane is placed near the transmitter.
(4) In the preferred embodiment of the present invention, the permeable attenuator is made of a highly permeable but substantially non-conductive material such as ferrite or mumetal. In the preferred embodiment, the thickness of the permeable layer when made of ferrite is from 0.05 inches to 0.25 inches whereas use of mumetal can reduce the thickness to below 0.01 inches. Of course, these ranges are merely exemplary. The thickness of the permeable attenuator is generally chosen such that the saturation flux density of the attenuator material is not exceeded. Some ferrites have a saturation flux density as low as a few hundred Gauss, while annealed iron materials have about 15,000 Gauss. Mu metal has a saturation flux density of about 7,000 Gauss. Analyzing the attenuator thickness combined with a transmitter means using finite element analysis will produce values for flux density within the attenuator. For relatively thin attenuators, flux density is inversely proportional to thickness, so if the density is seen to be at or near saturation, the attenuator can be made thicker. In other cases, the transmitter excitation may be reduced. If the flux density within the attenuator exceeds the saturation value, the shielding effect of the attenuator is reduced. In some applications in which cost or weight is placed at a premium, operation with a saturated attenuator may still be acceptable, as the attenuator will still exhibit reduced sensitivity to metallic objects adjacent to and below the transmitter compared to a non-attenuator equipped system. The conductive plate, preferably made of an aluminum alloy, may be from {fraction (3/16)} of an inch to xc2xc inch in thickness. In certain applications, it may be more efficient to employ the permeable attenuator without a conductive plate. In the case of DC transmitter excitation, for example, the additional shielding effect of the conductive plate is reduced to insignificant levels. In certain other cases, the performance benefit of the conductive plate may be outweighed by reduced mass, thickness, or other system considerations. In cases such as these, the additional mechanical support provided by the conductive plate may also be unnecessary, so that the conductive plate is removed entirely. In cases where the conductive plate provides a performance benefit to the system, this benefit is always of a secondary nature, with the primary performance enhancement arising from the permeable attenuator. Where mumetal is employed in the permeable layer, the thickness of the conductive plate may be reduced because the thickness is not chosen for mechanical support. Above the permeable attenuator, transmitter means are located. In one embodiment, the transmitter may consist of a PC board with the transmitter etched thereon.
(5) In certain applications, it may be more efficient to employ the permeable attenuator without a conductive plate. In the case of DC transmitter excitation, for example, the additional shielding effect of the conductive plate is reduced to insignificant levels. In certain other cases, the performance benefit of the conductive plate may be outweighed by reduced mass, thickness, or other system considerations. In cases such as these, the additional mechanical support provided by the conductive plate may also be unnecessary, so that the conductive plate is removed entirely. In cases where the conductive plate provides a performance benefit to the system, this benefit is always of a secondary nature, with the primary performance enhancement arising from the permeable attenuator.
(6) If a conductive object in the regions adjacent to or under the transmitter is subjected to an AC magnetic field, an eddy current will be induced in the object. This induced eddy current will produce a magnetic field component, which, by the addition of vectors, will combine with and distort the normal metal-free magnetic field near the object. The magnitude of this parasitic eddy field is proportional to the magnitude of the AC field near the conductive object.
(7) It is thus seen that if the field vectors in the operating volume above the transmitter assembly remain constant in magnitude and direction while the field magnitude in the regions adjacent to and under the transmitter assembly are reduced, then metallic objects in those regions will have a proportionally reduced distorting effect on the field in the operating volume above the transmitter assembly. If the field magnitude in the operating volume above the transmitter assembly is increased while the field magnitudes in the regions adjacent to and under the transmitter assembly remain constant, the distortion reducing effect is similar. Accordingly, the ratio of the magnetic field amplitude in the operating region above the transmitter assembly over that of the regions adjacent to and under the transmitter assembly may be used to predict sensitivity to metallic objects. A similar description applies to ferromagnetic distortion effects when the distorting objects are located in the regions adjacent to and below the transmitter assembly.
(8) If the relative magnetic distortion sensitivity values of a single transmit coil in the configuration such as is shown in FIG. 13, can be established as a normal value, then a relative distortion sensitivity figure of merit Ma for objects adjacent to the operating volume may be defined where Ma equals (the field of the system depicted in FIG. 2 in the region above the transmitter assembly) divided by (the field of the system depicted in FIG. 5 in the region above the transmitter assembly) divided by (the field of the system illustrated in FIG. 2 in the region adjacent the transmitter assembly) divided by (the field of the system in the configuration of FIG. 5 in the region adjacent the transmitter assembly). The system depicted in FIG. 11 will have a sensitivity figure of merit of 1 in that FIG. 11 will, for example, be chosen as the reference system.
(9) Similarly, for comparison of objects below the transmitter assembly, we can define a term Mb which equals (the field of the system of FIG. 2 in the region above the transmitter assembly) divided by (the field of the system illustrated in FIG. 5 in the region above the transmitter assembly) divided by (the field of the system of FIG. 2 below the transmitter assembly) divided by (the field of the system of FIG. 5 in the region below the transmitter assembly). Using the figures of merit Ma and Mb, several different configurations can be evaluated to determine likely relative sensitivities to metallic objects in the regions adjacent to and below the transmitter assembly.
Accordingly, it is a first object of the present invention to provide a magnetic position measurement system with field containment means.
It is a further object of the present invention to provide such a system wherein a permeable attenuator is provided.
It is a still further object of the present invention to provide such a system wherein the substantially high permeability, substantially non-conductive attenuator has upturned peripheral edges.
It is a still further object of the present invention to provide such a system wherein the substantially high permeability, substantially non-conductive attenuator has peripheral edges that taper downwardly from a top surface thereof to a bottom surface thereof.
It is a still further object of the present invention to provide such a system wherein transmitter means are mounted above the permeable attenuator.
It is a still further object of the present invention to provide a system for quantitatively measuring the position of receiving antennae relative to transmitting antennae without encountering the disadvantages that accrue from sensitivity to metallic objects directly below the transmitter.
It is a yet further object of the present invention to create a system that is insensitive to metallic objects at or below the plane of the transmitter.
It is a still further object of the present invention to provide such a system which avoids loss of transmit field intensity within the intended operating volume.
It is a still further object of the present invention to provide such a system which is not significantly degraded in performance by geometric dilution effects.
It is a yet further object of the present invention to provide such a system which may use either DC or AC transmitter excitation techniques and which is insensitive to magnetic objects placed below the transmitter configuration.